1. Field of the Invention
The present invention relates to a color processing method and a color processing apparatus and, more particularly, to a color processing method and a color processing apparatus for generating a color separation table used in an image forming apparatus which forms an image using color materials and a clear color material.
2. Description of the Related Art
As an ink for an inkjet recording apparatus, a dye ink containing a water-soluble dye as a color material is widely used. The color material dissolved in a solvent in the dye ink, whose main component is water, easily permeates into the fiber of a recording medium. This makes it easy to maintain the surface shape of the recording medium even after recording an image, and the gloss of the recording medium itself is kept as that of the image. In other words, an image excellent in gloss can be easily obtained by recording an image on a recording medium excellent in gloss using the dye ink. The inkjet recording apparatus using the dye ink can adjust the gloss of an image by adjusting that of a recording medium.
The dye ink is generally poor in lightfastness; the dye molecule of the color material is decomposed by light, and an image readily fades. In general, a material printed with the dye ink is also poor in water resistance; when it gets wet, the dye molecule permeated in the fiber is dissolved in water, and the image smears.
To solve the poor lightfastness and water resistance with the use of the dye ink, a pigment ink using a pigment as a color material has been developed recently. Unlike the dye present as a molecule, the pigment ink exists as a particle several ten nm to several μm in diameter in the solvent. The color material particle of the pigment ink is larger than that of the dye ink, and a printed material with high lightfastness and water resistance can be attained.
The color material of the pigment ink hardly permeates into a recording medium and accumulates on the surface of the recording medium. The fine shape of the image surface, therefore, differs between a region where the pigment ink is applied and a region where it is not applied. The amount of color material used changes depending on the density and color of an image. Hence, the area which the color material covers on the recording medium changes, the reflectance of the color material and the surface reflectance of the recording medium differ from each other, and thus the gloss varies owing to the difference in area which the color material covers on the recording medium.
For these reasons, recording an image using the pigment ink causes a phenomenon called “heterogeneity of glossiness” in which the gloss changes depending on the density and color of an image. Once heterogeneity of glossiness occurs, a glossy region observed to be glossy and a matte region observed not to be glossy coexist in a single image. Especially when the image is a photographic image, this causes the image to be recognized as a poor image.
The heterogeneity of glossiness occurs not only in an inkjet recording apparatus using pigment ink but also in an electrophotographic recording apparatus which records an image by fixing toner onto a recording medium. For example, in a region where the amount of applied toner is large, fixing makes the image surface greatly smooth. For plain paper with low glossiness, the glossiness becomes higher in a region where an image exists than in a region where no image exists and the surface of the recording medium is exposed. Note that it is known that the glossiness of a region where an image exists changes depending on the application amount, fixing temperature, and fixing speed of toner, and the like.
To suppress the heterogeneity of glossiness, a method using a substantially colorless invisible ink (to be referred to as a clear ink) which does not affect color reproduction is known. More specifically, the heterogeneity of glossiness is suppressed by applying a clear ink or white color ink to a region not covered with a color ink (for example, Japanese Patent Laid-Open No. 2002-307755 (reference 1)). There are also known a method of recording while keeping the total amount of color and clear inks constant regardless of the color or gray level to be reproduced, and a method of recording while keeping the total amount of color and clear inks constant in a region (to be referred to as a highlight portion) where the gray level is lower than a predetermined density (for example, Japanese Patent Laid-Open No. 2007-276482 (reference 2)).
Another method uses an ink (to be referred to as a semiclear ink) which has a faint color and is not substantially colorless (for example, Japanese Patent Laid-Open No. 2004-202790 (reference 3)). The semiclear ink has a characteristic in which a color reproduced using both the semiclear and color inks differs from one reproduced using only the color ink. When using the semiclear ink, first, a plurality of patches is printed at different recording amounts of semiclear and color inks. Then, combinations of semiclear and color inks which reproduce predetermined colors are obtained based on the colorimetric values of the patches, generating a color separation lookup table (to be referred to as a color separation table).
However, in the technique disclosed in reference 1, dots of the clear ink are formed at positions where they have an exclusive relationship with dots formed using a color ink. In other words, the entire surface of a recording medium is covered with ink regardless of whether the ink is a color or clear ink. This method does not use the clear ink at intermediate and higher densities at which the entire surface of a recording medium is covered with the color ink. At intermediate and higher densities, the amount of clear ink used may not be optimum for suppressing the heterogeneity of glossiness.
In the technique disclosed in reference 2, the total amount of color and clear inks is made constant regardless of the reproduction color and gray level. In some cases, the clear ink may be used more than necessary, or the amount of clear ink used may be insufficient to suppress the heterogeneity of glossiness depending on the characteristics (for example, surface roughness and surface reflectance) of each ink used that affect the gloss.
The technique disclosed in reference 3 can obtain the amount of semiclear ink used to achieve a desired reproduction color, but does not attain the amount of semiclear ink used to suppress the heterogeneity of glossiness.
In addition to the above-described heterogeneity of glossiness, recording apparatuses using the pigment ink and toner have the following problems.
Some types of pigment inks and toners used cause thin-film interference on a thin film formed by the pigment ink or toner on the surface of a recording medium. The thin-film interference occurs when the thickness of the thin film is equivalent to the wavelength of light and its surface is relatively smooth. If the thin-film interference occurs, specular reflection by a printed material (and light diffused near the angle of specular reflection) has various colors.
A printed material recorded with the pigment ink or toner generates reflection having wavelength dependence in accordance with the characteristics of a material exposed on the surface of the printed material. This is called a “bronze phenomenon”. Once the bronze phenomenon occurs, specular reflection by a printed material (and light diffused near the angle of specular reflection) is colored, similar to the thin-film interference. The observer observes specular reflection by a printed material as an illumination image reflected in the printed material. For this reason, if the thin-film interference or bronze phenomenon is generated, the color of an illumination image reflected in a printed material is observed as a color different from the original illumination color. In the following description, the phenomenon in which an illumination image reflected due to the thin-film interference is observed to have a color will be called a “structural color”, and the phenomenon in which an illumination image reflected owing to the bronze phenomenon is observed to have a color will be called a “bronze color”. Also, these colors will be generically called a “specular color”.
A printed material recorded with the pigment ink or toner has a different surface structure and material depending on the reproduction color and gray level. As described above, the thin-film interference and bronze phenomenon depend on the surface structure and material of a printed material, so a printed material whose surface structure and material change for each color or gray level has a different specular color for each color or gray level. The observer feels such a printed material to be unnatural because the specular color of an image region formed in a plurality of colors is observed to be different depending on the image position. This phenomenon will be called “heterogeneity of specular color”. Conventionally, image formation has not considered the specular color.
In some cases, the specular glossiness and image clarity will be generically called “gloss”, and the gloss and specular color will be called “gloss characteristic” at once.